A field test of attractant traps
for invasive Burmese pythons (Python molurus bivittatus) in southern Florida

Discussion

We successfully captured the
target species in traps with both entrance types, and our various detection
methods yielded an estimate of python density in the study area. However, an experimental trap trial with
a capture rate <0.05% per trap night cannot be considered particularly
successful in terms of potential control of an invasive species such as the
Burmese Python.

Standardised visual surveys
yielded no python observations, but two pythons were opportunistically observed
during trap checks. These results
confirmed our suspicion that diurnal visual surveys are unlikely to be
effective for pythons in summer, during which pythons appear to engage in
movements primarily during nocturnal hours; warm-season visual surveys
conducted by driving along roads at night have resulted in capture of several
hundred pythons in southern Florida (Snow, Mazzotti, unpubl. data). We also doubt that nocturnal pedestrian
surveys in summer using lights would be very effective, because a pedestrian
can cover only a small area in an evening and the thick vegetation present in
much of southern Florida would reduce detection rates. In contrast, diurnal surveys in the
winter months can yield much higher catch per unit effort along cleared levees
and other areas, because pythons emerge from concealing vegetation to bask. Continued accumulation of insights on
python activity and detection probability will allow land managers to focus
control efforts, including trap deployment, on times and places that will
maximise catch per unit effort.

Most large-bodied pythons (and
other giant constrictor snakes) appear to employ ambush predation as their primary
means of capturing prey (Branch 1988; Dirksen 2002; Reed et al. 2007; Reed and
Rodda 2009), although there are a few observations of active foraging (Martins
and Oliveira 1998; Alexander and Marais 2007). Ambush predation implies relatively infrequent movement,
which could reduce rates of encounters with control devices such as traps (Shiroma
and Akamine 1999). Moreover, an
ambush predator might approach the vicinity of an attractant traps, detect the
attractant scent, and settle into an ambush posture rather than actively
investigating the trap and encountering the entrance. For these and other reasons, attractant traps may be
generally ill-suited to capturing large numbers of ambush-foraging snakes. However, our results indicate that at
least some pythons entered traps, and we did not observe any pythons in ambush
postures adjacent to traps. Radiotelemetry
and other research avenues will be needed to determine daily and seasonal
movement behavior of pythons, and results may allow prediction of the
likelihood of pythons encountering attractant traps in various habitats and
trap densities. Such information
would be a vital component of planning for integrated python control efforts.

Native and introduced rodents
were abundant by the end of the study, likely supplying a ready prey resource
for pythons. Indeed, high rodent
biomass associated with high primary productivity and vegetable availability may
have drawn pythons into the Frog Pond area in previous years when the site was
in intensive agricultural production, and apparently high python density was a
primary reason for selecting this site for the trap experiment. High rodent abundance may have adversely
affected our python capture rates. Capture rates in snake traps often decline when prey are abundant (Gragg
et al. 2007; Tyrrell et al. 2009), possibly because snakes reduce foraging frequency
in a prey-rich environment and thus have fewer opportunities to encounter traps. Necropsies of pythons collected from
the Frog Pond in previous years revealed high predatory success, with as many
as 14 rats identified from a single python's gastrointestinal tract. In prey-rich environments, an ambush
predator may not need to change ambushing locations frequently, thus reducing
encounter rates with attractant traps. Scent from abundant prey may also mask the scent from attractant traps, further
reducing encounter rates.

We had expected that post-trapping
disc-harrowing operations would kill any pythons over which the harrow ran, but
instead we observed a high apparent survival rate (>60%) of 11 pythons found
during these operations. None of
the 11 pythons had previously been captured in a trap. Lack of recaptures could be due to behavioral
responses in previously-trapped pythons, including the tendency to avoid traps (trap
shyness) or because they fled the study site after capture. Alternatively, trapped pythons may have
emigrated from the study site during the experiment as part of their regular
pattern of activity. Based on
their body sizes (1740 - 2240 cmSVL), the lack of subterranean refugia in the
area that was harrowed, and the thoroughness of our searching behind the
harrow, we believe it unlikely that the three marked pythons were killed by the
harrow and missed by observers.

Disc-harrowing results have implications
for previous estimates of python density in the Frog Pond area. In previous years, python density
estimates ranged from 0.044 to 0.109/ha across a large (505 ha) area, but these
were largely based on finding dead pythons in the fields, often several days
after harrowing, with python presence indicated by the presence of vultures. If
our finding that 7 of 11 pythons actually survived harrowing is a reasonable
estimate of survival rate, adjustment of results from previous years in the
Frog Pond would yield a range of 0.069-0.171 pythons/ha. This range is comparable to the estimate
of 0.136/ha we derived from harrowing 81 ha in 2009. However, these are minimum density estimates; disc-harrowing
occurred over a number of days and some pythons may have left the study site in
response to mechanical disturbance and vibration resulting from nearby
harrowing activities, or we may not have detected some pythons killed and
buried by the harrow.

It is possible that both rodent
and python densities were relatively low in April 2009 when the fields were
allowed to go fallow, and rodent densities probably increased exponentially in
ensuing months in response to rapid growth of vegetation in the fields. If pythons select habitats with high
prey abundance, then it follows that python density would have increased in the
Frog Pond area in response to increasing rat density. Python densities may thus have been low during early parts
of the trap experiment, rendering difficult any attempt to estimate trap
success over the duration of the trap experiment as a function of the number of
pythons present.

Our traps captured relatively
few non-target organisms, and survival of non-targets was 100%. Conservation concerns associated with deaths
of native non-targets captured in python traps may be minimal, with the caveat
that results are likely to vary across habitats with different animal
communities. If conservation concerns are allayed, then it may be possible to
check traps at less-frequent intervals; labor is typically the most expensive component
of trapping budgets and fewer trap checks per unit time would greatly reduce
costs associated with operational python trapping.

There were major discrepancies
in the perception of non-target species composition and relative abundance resulting
from visual surveys, trap captures, and disc-harrowing in the Frog Pond area. Disc-harrowing exposed 18 rat snakes (Pantherophis), for example, but none
were captured in traps or observed during visual surveys. Conversely, 24 amphibians were captured
in traps but only 6 were observed during visual surveys and none were observed
during harrowing. The innate
biases associated with any detection or capture method must be considered when
attempting to assess species composition or relative abundances, and any single
method will likely yield biased results.

Our field experiment resulted
in few captures of the target species, despite a moderately large effort and a
pool of available targets that at any given time probably exceeded the number
captured. Behavioral (e.g., ambush
predation) or environmental (e.g., high prey density) factors may have reduced
the likelihood that a python would encounter, be attracted to, and enter our
traps. Alternatively, our traps
may be suboptimal in their design or choice of attractant, potentially failing
to attract pythons or frustrating their efforts to enter. Additional research would be invaluable
in helping to distinguish between these hypotheses. However, developing a snake trap that maximises capture
rates can be a lengthy process. For example, field tests of 49 different trap designs and >24,000
trap-nights were required to settle on an optimal design for capture of brown treesnakes
on Guam (Rodda et al. 1999b), yet even the less effective designs often had
capture rates approximately two orders of magnitude higher than we achieved for
pythons. We anticipate that a
series of additional experiments in both field and captive settings will be
required for development of an optimal python trap. As an example, unbaited traps placed along drift fences (Jackley
1943; Gibbons and Semlitsch 1981) and other methods designed to intercept
moving pythons may capture individuals that are not motivated by prey (e.g,
aphagic individuals during the mating season), as well as reducing bycatch of
non-target predators attracted to bait in attractant traps.

There are now three species of
exotic giant constrictors established in Florida (Snow et al. 2007a; Reed et
al. 2010) and suitable climatic conditions for other species of giant
constrictors appear to exist in the United States (Reed and Rodda 2009), so
land managers will continue to request effective control tools for these taxa. Our results suggest that traps are
unlikely to result in eradication of pythons at landscape scales, but that additional
testing may result in traps that are a useful component of management efforts
that include a range of control tools. More generally, our results highlight the difficulty of developing control
tools for invasive top predators that are cryptic and which may have behaviors
that make them unlikely to regularly encounter control tools. Overall, we conclude that refinement of
trap technologies and further assessment of various environmental contributors
to trap success will be necessary before it will be possible to produce a
thorough assessment of the utility of traps for invasive giant constrictor
snakes.